In current optical amplifier assemblies for use in submarine applications, a Gain Flattening Filter (GFF) is inserted at the output of the second fiber amplifier of the individual double stage fiber amplifiers comprised in the optical amplifier assembly in order to flatten the gain of the double stage fiber amplifier without degradation of the noise figure (NF) of the amplifiers. In this case, however, the output power of the double stage fiber amplifier is reduced by the filter (insertion loss).
If high gain (>16 dB) and wide transmission bandwidth (27 nm or more) are considered, the induced insertion loss due to prior art optical amplifier assemblies becomes extremely large. Thus, in the prior art, the output power of optical amplifier assemblies, i.e. of the individual double stage fiber amplifiers comprised therein, is limited to only moderate power levels. Alternatively, pump power required to reach higher output power levels becomes extremely high, such that an excessively large number of pump light sources, e.g. laser diodes, has to be employed which leads to a reduced cost-effectiveness of the overall system.
For future applications, it is expected that the span length, i.e. the distance between successive optical amplifier assemblies in submarine optical transmission systems (and thus a corresponding span loss), will continue to increase in order to achieve a reduction of the number of amplifiers in the transmission system, leading to a reduced total system cost. In addition, an upcoming generation of optical fibres with extremely large effective areas (up to 200 μm2) could be responsible for an increase in span loss of about 4 dB, which will have to be compensated by means of a corresponding EDFA output power increase in order to maintain the performance characteristics of current optical transmission systems.
Furthermore, an increase in system bandwidth to 32 nm or even 38 nm has to be expected in the future. As a result to these system modifications, induced losses of the GFF will increase drastically. However, for the reasons indicated above, it will not be possible to increase the output power of prior art optical amplifier assemblies accordingly.
The majority of submarine optical transmission systems in use today have relatively moderate span losses (maximum 15 dB), and some of them have a wide bandwidth (27 nm). Systems with shorter reach and corresponding higher span loss (up to 30 dB) use narrow bandwidth amplifiers (<17 nm) and do not require gain flattening filters. In the context of the present invention, if one considers high span losses in connection with wide bandwidth amplifiers, a solution proposed in prior art transmission systems is to increase the pump power by several decibels in order to compensate the extra loss of the GFF, or to use amplifiers with only low output power, which obviously does not lead to an optimum system performance. However, if the output power of amplifiers used in optical transmission systems is too low, the overall system performance can only be increased to an acceptable level by adding additional amplifiers to the system in order to reduce span loss. Such an approach, however, will lead to an increase in overall system costs. On the other hand, increasing the output power by providing powerful pump light sources suffers from the fact, that pump power higher than 500 mW is not available. Simply providing an ever increasing number of individual pump light sources will again have a negative effect on overall system cost.